Sputter deposition of refractory carbide on metal working

ABSTRACT

A METHOD OF COATING METALWORKING TOOLS WITH A THIN UNIFORM ADHERENT LAYER OF REFRACTORY CARBIDE, E.G., TITANIUM CARBIDE, IS DISCLOSED. SUCH COATING IS APPLIED BY TRIODE SPUTTER DEPOSITION USING A REFRACTORY CARBIDE CATHODE AND A TOOL SUBSTRATE.

United States Patent 3,709,809 SPUTTER DEPOSITION 0F REFRACTORY CARBIDE0N METAL WORKING Robert J. Wright, Arvada, John S. Chapin and GeorgeMah, Boulder, and Claude A. Karstendiek, Arvada, Colo., assignors to TheDow Chemical Company, Midland, Mich.

Filed July 15, 1971, Ser. No. 162,778 Int. Cl. C23c 15/00 US. Cl.204-192 12 Claims ABSTRACT OF THE DISCLOSURE A method of coatingmetalworking tools with a thin uniform adherent layer of refractorycarbide, e.g., titanium carbide, is disclosed. Such coating is appliedby triode sputter deposition using a refractory carbide cathode and atool substrate.

The invention described herein was made in the course of or under acontract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION The coating of metalworking tools withtitanium carbide greatly improves cutting, extruding and stampingproperties of the tools. In use, these coated tools result in areduction of cutting force and temperature rise which mean less wear andbetter surface finish. The high hardness and thermal conductivity oftitanium carbide and other refractory carbides also contribute tosuperior cutting properties.

A conventional method of applying such a refractory carbide coating isthat of chemical vapor deposition. For example, titanium tetrachlorideis reacted with methane to produce titanium carbide and hydrochloricacid. The titanium carbide so formed is deposited out on the cuttingtool surface. While providing an adherent coating of the refractorycarbide on the cutting tool, the chemical vapor deposition process hassome drawbacks. Hydrochloric acid is a byproduct of the process andpresents a serious corrosion problem. The substrate is known to pick uphydrogen and for many metals this can cause embrittlement. The processrequires the cutting tool substrate to be maintained at temperatures of900 to 1200 C. which limits the materials which can be coated. Also, theprocess takes a relatively long period of time, e.g. 8-10 hours.

It is an object of the present invention to provide a method of coatingmetalworking tools with a thin, uniform, adherent refractory carbide.

It is a further object of the present invention to provide a method ofcoating refractory carbides on tools which avoids the hydrochloric acidby-product, reduces hydrogen pickup, operates at lower temperatures andshorter times.

THE INVENTION The present method is based upon the discovery thatrefractory carbides, such as titanium carbide, can be effectively coatedon metalworking tool substrates by triode sputter deposition. Uniformadherent coatings on the order of from about 2 to about microns can beapplied to the tools using the present technique employing temperaturesof from about 400800 C. and times of about 2 to about 6 hours.

The present process involves providing, in a vacuum chamber, arefractory carbide target cathode, a metalworking tool substrate and athermionic electrode; degassing the tool substrate; sputter cleaning thesubstrate lCC and target cathode surfaces; and sputter depositing thetarget material onto the cutting tool substrate.

The figure is a drawing of one embodiment of equipment that can beutilized in the present method.

The metalworking tool substrate may be any particular tool used to cutor otherwise plastically deform materials. Such tools include tool bits,drill bits, cutting tool mserts, stamping tools and dies, extrudingtools and dies, forming tools and dies, blades, files, and planes.Conventionally such tools are made from various tool steels or tungstencarbide. When tungsten carbide is used as a cutting tool substrate it ispreferred that a post deposition heat treatment be carried out on suchtools to stress relieve the coating.

In carrying out the present method the target cathode and substrate arepositioned within a metal vacuum chamber opposite each other with thethermionic electrode disposed therebetween. Usually the target cathodeis in the form of a rectangular or circular plate and the thermionicelectrode in the form of a wire positioned between or about the targetcathode and the tool substrate.

After a vacuum is drawn on the chamber containing the electrodes, thethermionic electrode is heated by applying a potential across the wire.A positive electrical bias is applied to the tool substrate to causeelectrons from the heated electrode to bombard and heat the substrate.The cutting tool substrate is heated to a temperature of from about400800 C. whereby gases are removed from the tool. This step is carriedout to enhance the adhesion between the cutting tool and thesubsequently deposited cathode material.

Following the degassing process a low pressure gas is introduced intothe chamber. Any gas which is inert, i.e. unreactive with the materialsof the cathode, substrate and thermionic electrode can be employed. Atypical gas is argon.

The surfaces of the target cathode and substrate are then cleaned byapplying a negative bias to the target cathode and the substrate,creating anions of the gas, and causing the gas anions to bombard thetarget cathode and substrate surfaces removing the surface layers. Theamount of negative bias is preferably from about 1 to about 3 kilovolts.After the cleaning operation the amount of gas entering the chamber isreduced from about 10 to about 20 microns down to about 1 to about 5microns. Such gas is ionized by electrons emitted from the thermionicelectrode. A negative bias is now applied only to the target cathode,with the vacuum chamber at ground potential, to cause deposition of therefractory carbide on the cutting tool substrate. A negative bias isapplied to the thermoionic electrode. Preferably, the negative bias onthe target cathode is from about 0.5 to about 2 kilovolts and thenegative bias on the thermionic electrode is from about 50 to aboutvolts. A bias can be applied to the substrate for control of thestructure of the coating.

As noted, where tungsten carbide is the substrate mate rial it isadvantageous to heat treat the coated cutting tool by, for example,applying a positive bias to the coated substrate to cause electrons fromthe thermionic electrode to bombard the coated substrate. Heating thecoated material for about one-half hour at 400-800 C. stress relievesthe coating.

Cutting tools when treated in the prescribed manner have excellentcutting properties of wear resistance, tool force and surface finish. Auniform adherent coating of refractory carbide is achieved.

The figure describes an apparatus which can be employed in the presentmethod. Such apparatus comprises a vacuum chamber 1 with a cover 2sealed by gasket 3 and openings for gas inlet and vacuum control;metalworking tool substrates 4 and substrate holder 5 insulated byinsulators 6 and connected to a substrate power supply; thermionicelectrode filament 7 and filament posts 8 connected to filament andfilament bias power supplies; and refractory carbide target 9 connectedto target power supply, water-cooled target holder 10 and target shield11.

In carrying out the present method, in the vacuum chamber the thermionicelectrode 7 is heated and a positive bias applied to the tool substrate4 through the substrate holder 5 to degas the substrate. After turningoff electrode 7 argon or other inert gas is introduced into the chamberat low pressure and a negative bias applied to cathode 9 and toolsubstrate 4 to create gas anions and cause the anions to bombard thecathode and substrate, thereby sputter cleaning the surfaces. Sputteringof the target material on substrate 4 is accomplished by reducing thegas flow (the incoming gas being ionized by the electrons emitted fromthe heated thermionic electrode 7 which is again turned on), andapplying a negative bias on cathode 9. To achieve more adherent coatingswith refractory coated tools, the final product may be stress relievedby bombarding the newly deposited surface of positively biased substrate9 with electrons from heated thermionic electrode 7.

EXAMPLES A vacuum chamber similar to the one depicted in the figure wasbuilt with openings for introduction of the ionizable gas andelectrodes. A 6 inch diameter titanium carbide target was fitted into anopening in the top of the vacuum chamber with connections to a powersupply and cooling system. Disposed below the titanium carbide targetwere several cutting tools placed on a substrate holder which wasconnected to a power supply. The metal vacuum chamber was used as theanode at ground potential during the deposition. A circular tungstenfilament was inserted between the titanium carbide target and thecutting tool substrate. Both tungsten carbide and hardened tool steelwere employed as substrate materials. A shield was placed around theedge of the titanium carbide target to prevent sputtering of the targetholder. Argon was employed as the ionized gas. Coating of the cuttingtools was carried out using the bias parameters and process steps asherein disclosed. Substrate temperatures ranged from about 400 to about650 C. during the deposition. Adherent coatings on the order of 510microns of titanium carbide were deposited on the substrate at adeposition rate of about 2 to about 4 microns per hour.

In like manner other refractory carbides can be sputter deposited onvarious cutting tool surfaces. Such refractory carbides include siliconcarbide, vanadium carbide, chromium carbide, zirconium carbide,molybdenum carbide, tantalum carbide, and other heavy metal and rareearth metal carbides. Also, other metal alloys possessing desirablecutting properties can be sputter deposited on cutting tool surfaces.

What is claimed is:

1. A method of coating a refractory carbide on a metalworking toolsubstrate which comprises:

(a) positioning within a vacuum chamber a refractory carbide targetcathode and a metalworking tool substrate in oppositely disposed spacedrelationship, and disposed therebetween a thermionic electrode;

(b) heating the thermionic electrode;

(c) applying a positive bias to the substrate anode to cause electronsfrom the heated electrode to bombard and heat the substrate, therebydegassing the substrate;

(d) introducing a low pressure gas into the chamber after turning offthe thermionic electrode;

(e) sputter cleaning the surfaces of the target cathode and thesubstrate by applying a negative bias to said cathode and substrate tocreate gas anions and to cause the gas anions to bombard the cathode andsubstrate surfaces;

(f) reducing the amount of gas entering the chamber;

(g) reheating the thermionic electrode and applying a negative bias tosaid electrode;

(h) ionizing the incoming gas by electrons emitted from the negativelybiased thermionic electrode; and

(i) applying a negative bias on the cathode with the vacuum chamber atground potential to sputter de posit a coating of refractory carbide onthe cutting tool substrate.

2. The method of claim 1 wherein the refractory carbide is titaniumcarbide.

3. The method of claim 1 wherein the thermionic electrode is tungsten.

4. The method of claim 1 wherein low pressure gas is argon.

5. The method of claim 1 wherein in step (c) the positive bias appliedto the substrate is from about 0.5 to about 3 kilovolts.

6. The method of claim 1 wherein in step (d) the gas is introduced at apressure in an amount of from about 10 to about 20 microns.

7. The method of claim 1 wherein in step (e) the negative bias appliedto the substrate is from about 1 to about 2 kilovolts and to the targetcathode is from about 2 to about 3 kilovolts.

8. The method of claim 1 wherein in step (f) the gas is reduced to apressure of about 1 to about 5 microns from a pressure of about 10 to 20microns.

9. The method of claim 1 wherein in step (i) the negative bias on thethermionic electrode is from about 50 to about volts.

10. The method of claim 1 wherein in step (i) the negative bias on thetarget cathode is from about 0.5 to about 2 kilovolts.

11. The method of claim 1 wherein the coating of refractory carbide onthe tool substrate has a thickness of from about 5 to about 10 microns.

12. The method of claim 1 wherein the tool substrate is made of tungstencarbide and including the additional step of heat treating the coatedsubstrate by applying a positive bias to the coated substrate to causeelectrons from the heated electrode to bombard the coated substrate.

References Cited UNITED STATES PATENTS 2/1967 Wehner 204-192 6/1969Moseson 204192 OTHER REFERENCES JOHN H. MACK, Primary Examiner S. S.KANTER, Assistant Examiner US. Cl. X.R. 204298

